1. Field of the Invention
The invention generally relates to a projection objective of a microlithographic projection exposure apparatus, and in particular to such an objective containing a wavefront correction device in which heating light distinct from projection light is directed towards a rim surface of a refractive optical element.
2. Description of Related Art
Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens. A typical value for the magnification is β=±¼.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning mechanism. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the apparatus.
The size of the structures which can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a wavelength of about 13 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6.9 nm.
Another way of increasing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an immersion interspace, which remains between a last lens on the image side of the projection objective and the photoresist or another photosensitive surface to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.3 or even higher.
The correction of image errors (i.e. aberrations) is becoming increasingly important for projection objectives with very high resolution. Different types of image errors usually require different correction measures.
The correction of rotationally symmetric image errors is comparatively straightforward. An image error is referred to as being rotationally symmetric if the wavefront error in the exit pupil of the projection objective is rotationally symmetric. The term wavefront error refers to the deviation of a wave from the ideal aberration-free wave. Rotationally symmetric image errors can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those image errors which are not rotationally symmetric is more difficult. Such image errors occur, for example, because lenses and other optical elements heat up rotationally asymmetrically. One image error of this type is astigmatism.
A major cause for rotationally asymmetric image errors is a rotationally asymmetric, in particular slit-shaped, illumination of the mask, as is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illuminated field causes a non-uniform heating of those optical elements that are arranged in the vicinity of field planes. This heating results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their refractive index. If the materials of refractive optical elements are repeatedly exposed to the high energetic projection light, also permanent material changes are observed. For example, a compaction of the materials exposed to the projection light sometimes occurs, and this compaction results in local and permanent changes of the refractive index. In the case of mirrors the reflective multi-layer coatings may be damaged by the high local light intensities so that the reflectance is locally altered.
The heat induced deformations, index changes and coating damages alter the optical properties of the optical elements and thus cause image errors. Heat induced image errors sometimes have a twofold symmetry. However, image errors with other symmetries, for example threefold or fivefold, are also frequently observed in projection objectives.
Another major cause for rotationally asymmetric image errors are certain asymmetric illumination settings in which the pupil plane of the illumination system is illuminated in a rotationally asymmetric manner. Important examples for such settings are dipole settings in which only two poles are illuminated in the pupil plane. With such a dipole setting, also the pupil planes in the projection objective contain two strongly illuminated regions. Consequently, lenses or mirrors arranged in or in the vicinity of such an objective pupil plane are exposed to a rotationally asymmetric intensity distribution which gives rise to rotationally asymmetric image errors. Also quadrupole settings often produce rotationally asymmetric image errors, although to a lesser extent than dipole settings.
In order to correct rotationally asymmetric image errors, U.S. Pat. No. 6,338,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced image errors are at least partially corrected. A more complex type of such a correction device is disclosed in US 2010/0128367 A1.
U.S. Pat. No. 7,830,611 B2 discloses a similar correction device. In this device one surface of a deformable plate contacts an index matched liquid. If the plate is deformed, the deformation of the surface adjacent the liquid has virtually no optical effect. Thus this device makes it possible to obtain correcting contributions from the deformation not of two, but of only one optical surface. A partial compensation of the correction effect, as it is observed if two surfaces are deformed simultaneously, is thus prevented.
However, the deformation of optical elements with the help of actuators has also some drawbacks. If the actuators are arranged at the circumference of a plate or a lens, it is possible to produce only a restricted variety of deformations with the help of the actuators. This is due to the fact that both the number and also the arrangement of the actuators are fixed. In particular it is usually difficult or even impossible to produce deformations which may be described by higher order Zernike polynomials, such as Z10, Z36, Z40 or Z64. The aforementioned U.S. Pat. No. 7,830,611 B2 also proposes to apply transparent actuators directly on the optical surface of an optical element. However, it is difficult to keep scattering losses produced by the transparent actuators low.
US 2010/0201958 A1 and US 2009/0257032 A1 disclose a correction device that also comprises two transparent optical elements that are separated from each other by a liquid layer. However, in contrast to the device described in the aforementioned U.S. Pat. No. 7,830,611 B2, a wavefront correction of light propagating through the optical elements is not produced by deforming the optical elements, but by changing their refractive index locally. To this end one optical element may be provided with heating stripes that extend over the entire surface. The liquid ensures that the average temperatures of the optical elements are kept constant. Although even higher order wavefront errors can be corrected very well, this device has a complex structure and is therefore expensive.
Unpublished international patent application PCT/EP2010/001900 discloses a correction device in which a plurality of fluid flows emerging from outlet apertures enter a space through which projection light propagates during operation of the projection exposure apparatus. A temperature controller sets the temperature of the fluid flows individually for each fluid flow. The temperature distribution is determined such that optical path length differences caused by the temperature distribution correct wavefront errors.
Another, and sometimes simpler, approach to deal with heat induced image errors is not to correct errors that have been produced in a plurality of optical elements, but to avoid that such errors occur altogether. This usually involves the locally selective heating or cooling of optical elements so that their temperature distribution becomes at least substantially symmetrical. Any residual heat induced image error of the rotationally symmetric type may then be corrected by more straightforward measures, for example by displacing optical elements along the optical axis.
The additional heating or cooling of optical elements may be accomplished by directing a hot or cool gas towards the element, as it is known from U.S. Pat. No. 6,781,668 B2, for example. However, it is difficult to accurately control the temperature distribution of the optical element with gas flows.
Therefore it has been proposed to direct light beams onto selected portions of optical elements so as to achieve an at least substantially rotationally symmetric temperature distribution on or in the optical element. Usually the light beam is produced by a separate light source which emits radiation having a wavelength that is different from the wavelength of the projection light. The wavelength of this additional light source is determined such that the correction light does not contribute to the exposure of the photoresist, but is still at least partially absorbed by the optical elements or a layer applied thereon.
EP 823 662 A2 describes a correction system of this type in which two additional light sources are provided that illuminate the portions of the mask which surround the (usually slit-formed) field that is illuminated by the projection light. Thus all optical elements in the vicinity of field planes are subjected to three different light beams that heat up the optical element almost in a rotationally symmetrical manner. In other embodiments additional correction light is coupled into the illumination system of the projection exposure apparatus in or in close proximity to a pupil plane. Since, depending on the illumination setting, the center of the pupil plane is often not illuminated during the projection operation, light coupled into this center contributes to a more homogeneous illumination of optical elements that are arranged in or in proximity to a pupil plane in the projection objective.
U.S. Pat. No. 7,817,249 B2 discloses a device which directs heating light simultaneously on selected portions of two opposite lens surfaces. In one embodiment heating light produced by a heating light source is distributed by a spatial light modulator among eight optical fibers. Focusing optics associated with each optical fiber direct the heating light emitted by the optical fibers towards the selected portions of the lens surfaces.
US 2005/0018269 A1 describes a similar correction device which makes it possible to heat up certain portions of selected optical elements using a light ray that scans over the portions to be heated up. This device can also be arranged within the projection objective and makes it possible to increase the temperature very selectively so that an almost perfectly rotationally symmetric temperature distribution can be achieved.
If the correction device is arranged inside the projection objective, access to its optical elements is often restricted, and even if it is possible to direct heating light on all points on an optical element, the heating light often impinges on the optical surface at very large angles of incidence. As a result, a substantial fraction of the light energy is reflected at the surface and cannot contribute to the heating up of the elements.
In one embodiment described in the aforementioned US 2005/0018269 A1 this problem is solved in that the correction light passes through a plurality of optical elements without being subject to substantial absorption before the correction light impinges on the optical element which shall be heated up. This can be achieved by selecting materials for the optical elements which have a different coefficient of absorption for the correction light on the one hand and the projection light on the other hand. However, it is still difficult to reach all points of interest on an optical surface with a scanning light ray that passes through a plurality of other lenses before it impinges on the optical surface.
US 2010/0231883 A1 overcomes this problem by providing a correction device that includes a secondary illumination system which produces an intensity distribution of correaction light in a reference surface. This reference surface is imaged, using at least a portion of the projection objective, on a plane in which a refractive optical element is arranged. All lenses through which both the correction light and the projection light pass are made of a lens material which has a lower coefficient of absorption for the correction light than the material of the refractive optical element.
U.S. Pat. No. 6,504,597 B2 proposes a correction device in which heating light is coupled into a selected optical element via its peripheral rim surface, i.e. circumferentially. Optical fibers may be used to direct the heating light produced by a single light source to the various locations distributed along the periphery of the optical element. It is also mentioned that this device may not only be used to homogenize the temperature distribution of the optical element, but also to correct wavefront errors caused in other optical elements. Although this device makes it possible to heat up also optical elements that are very densely stacked, it is only capable to produce comparatively coarse temperature distributions. More complex temperature distributions cannot be attained because only very few and strongly divergent heating light beams can be coupled into the optical element.